Background: One of the most important concerns in orthopedic medicine is the low back. Considering the importance of muscle function in preventing LBT by controlling too much load and stress applied on the spinal joints and ligaments.

Materials and Methods: The aim of this research was to determine the timing and level of activities of lumbopelvic muscles in response to postural perturbations caused by unexpected loading of the upper limbs in standing on three different supporting surfaces (neutral, positive slope, negative slope) in 20 healthy females 18 to 30 years old ( = 23.20 SD = 2.55 ). The electromyographic signals were recorded from the deltoid, gluteus maximus, internal oblique abdominis and lumbar paraspinal muscles of the dominant side of the body to evaluate the onset time, end time, level of muscle activity (RMS) and duration of different muscles in one task and one muscle in different tasks.

Results: The results showed that the agonists (posterior muscles) activated at first to compensate the flexor torque caused by loading and then the antagonists (anterior muscles) switched-on to compensate the reaction forces caused by agonist activities. With regards to continuous activity of internal oblique and its attachments via thoracalumbar fascia to the transverse processes of the lumbar vertebrae, it can be considered as one of the major stabilizer muscles of the trunk .

Conclusion: Finally the results indicated that supporting surface type didn’t have any effect on timing and scaling of muscle activities in different tasks suggesting that probably spinal and trunk priprioceptors are just responsible for triggering postural responses and they don’t have any role in determining timing and scaling.